1. Field of the Invention
The present invention relates to a rotor shaft for a high-strength steam turbine fit for a new thermal power plant, more particularly fit for a ultra-super critical pressure thermal power plant.
2. Description of the Prior Art
For higher efficiency of power generation, the recent thermal power plants have used higher temperature and higher pressure. For example, recent steam turbines use steam of about 600 degrees C which is the highest steam temperature at present. In the near future, steam of 650 degrees C will be used. To use steam of such a high-temperature, the conventional heat-resisting ferrite steel must be substituted by high heat-resisting materials whose strength is excellent at such a high temperature. Some of austenite heat-resisting alloys are superior at such a high temperature, but their thermal fatigue strengths are inferior because their coefficients of thermal expansion are very big.
Some examples of new heat-resisting ferrite steel whose strength at a high temperature is improved are disclosed by Japanese Non-examined Patent Publications No.04-147948(1992) and No.08-30249(1996).
However, when used for a long time at an extremely high steam temperature of 650 degrees C, these proposed alloys containing much tungsten produce fragile intermetallic compounds which reduce the long-term creep rupture strength. Therefore, these alloys are still not perfect as materials to be used at an extremely high vapor temperature. The world has expected high heat-resisting ferrite steel whose strength is extremely stable for a long time at such a high temperature.
The main object of the present invention is to provide a rotor shaft for a high-temperature steam turbine which is very strong for a long time at a selected temperature of 650 degrees or above.
It is an object of the present invention to provide a steam turbine rotor shaft comprising martensite steel containing 0.05% to 0.20% by weight of carbon, 0.20% or less by weight of silicon, 0.05% to 1.5% by weight of manganese, 0.01% to 1.0% by weight of nickel, 9.0% to 13.0% by weight of chrome, 0.05% to 0.5% by weight of molybdenum, 0.5% to 5.0% by weight of tungsten, 0.05% to 0.30% by weight of vanadium, 0.01% to 0.20% by weight of niobium, 0.5% to 10.0% by weight of cobalt, 0.01% to 0.1% by weight of nitrogen, 0.001% to 0.030% by weight of boron, and 0.0005% to 0.006% by weight of aluminum, wherein the remaining parts of this steel substantially comprise iron.
It is another object of the present invention to provide a steam turbine rotor shaft comprising martensite steel containing 0.09% to 0.15% by weight of carbon, 0.03% to 0.15% by weight of silicon, 0.35% to 0.65% by weight of manganese, 0.02% to 0.5% by weight of nickel, 9.5% to 11.5% by weight of chrome, 0.05% to 0.4% by weight of molybdenum, 1.0% to 3.0% by weight of tungsten, 0.15% to 0.30% by weight of vanadium, 0.04% to 0.13% by weight of niobium, 1.5% to 3.5% by weight of cobalt, 0.01% to 0.04% by weight of nitrogen, 0.005% to 0.025% by weight of boron, and 0.0005% to 0.005% by weight of aluminum; wherein the remaining parts of this steel substantially comprise iron.
It is yet a further object of the invention to provide a steam turbine rotor shaft comprising martensite steel containing 9.0% to 13.0% by weight of chrome, 0.5% to 5.0% by weight of tungsten, 0.05% to 0.30% by weight of vanadium, 0.01% to 0.20% by weight of niobium, 0.5% to 10.0% by weight of cobalt, and 0.001% to 0.030% by weight of boron; wherein the creep rupture strength at 650 degrees C for 100,000 hours is 9 kg/mm2 or more preferentially 10 kg/mm2. More preferentially said rotor shaft comprises a martensite steel having the above composition and the creep rupture strength for 100,000 hours is 20 kg/mm2 or more at 600 degrees C, 14 kg/mm2 or more at 625 degrees C, 25 kg/mm2 or more at 700 degrees C, more preferentially 22 kg/mm2 or more at 600 degrees C, 16 kg/mm2 or more at 625 degrees C, 3 kg/mm2 or more at 700 degrees C.
It is a more particular object of the invention to provide a steam turbine rotor shaft comprising martensite steel containing 0.09% to 0.15% by weight of carbon, 0.15% or less by weight of silicon, 0.3% to 0.7% by weight of manganese, 0.02% to 0.5% by weight of nickel, 9.0% to 13.0% by weight of chrome, 0.05% to 0.5% by weight of molybdenum, 1.0% to 3.0% by weight of tungsten, 0.15% to 0.30% by weight of vanadium, 0.04% to 0.13% by weight of niobium, more than 1.5% to 4.0% or less by weight of cobalt, 0.01% to 0.04% by weight of nitrogen, 0.001% to 0.030% by weight of boron, and 0.0005% to 0.006% by weight of aluminum; wherein the remaining parts of this steel substantially comprise iron.
It is still another object of the invention to provide a steam turbine rotor shaft comprising martensite steel containing 0.05% to 0.20% by weight of carbon, 0.20% or less by weight of silicon, 0.05% to 1.5% by weight of manganese, 0.01% to 1.0% by weight of nickel, 9.0% to 13.0% by weight of chrome, 0.05% to 2.0% by weight of molybdenum, 0.5% to 5.0% by weight of tungsten, 0.05% to 0.30% by weight of vanadium, 0.01% to 0.20% by weight of niobium, more than 1.0% to 10.0% or under by weight of cobalt, 0.01% to 0.1% by weight of nitrogen, 0.001% to 0.030% by weight of boron, and 0.0005% to 0.006% by weight of aluminum; wherein the remaining parts of this steel substantially comprise iron.
Carbon is an indispensable element to assure quenching, to separate carbides of M23C6 in the tempering processes, and thus to increase the high-temperature strength of the steel. The martensite steel of the present invention requires a minimum of 0.05% by weight of carbon.
More than 0.20% by weight of carbon causes excessive separation of M23C6 carbides and reduction of the degree of matrix. This lessens the high-temperature strength of the steel when used for a long time. The martensite steel of the present invention uses 0.05% to 0.20% by weight of carbon, more preferentially 0.09% to 0.13%.
Manganese(Mn) suppresses production of xcex4 ferrite and accelerates separation of carbides of M23C6. The martensite steel of the present invention requires a minimum of 0.05% by weight of manganese, usually 0.02% to 1.5%, preferentially 0.3% to 0.7%, more preferentially 0.35% to 0.65%. 1.5% by weight or above of manganese in the steel will reduce the resistance to oxidization. Nickel(Ni) suppresses production of xcex4 ferrite and adds toughness to the steel. The martensite steel of the present invention requires a minimum of 0.05% by weight of nickel.
More than 1.0% by weight of nickel reduces the creep rupture strength at 620 degrees C or above. The martensite steel of the present invention uses 0.02% to 1.0% by weight of nickel, more preferentially 0.1% to 0.5%.
Chrome(Cr) is an indispensable element to increase the resistance to oxidization of the steel, to separate M23C6 carbides, and thus to increase the high-temperature strength of the steel. The martensite steel of the present invention requires a minimum of 9% by weight of chrome.
More than 13% by weight of chrome causes production of xcex4 ferrite and reduces strength and toughness at high temperatures. The martensite steel of the present invention uses 9.0% to 13.0% by weight of carbon, preferentially 9.5% to 11.5%, more preferentially 10.0% to 11.0%.
Molybdenum(Mo) accelerates separation of fine particles of M23C6 carbides and prevents them from cohering. Consequentially, molybdenum Mo is effective to give high strength at a high temperature for a long time. The martensite steel of the present invention requires a minimum of 0.05% by weight of molybdenum. 2% or above by weight of molybdenum causes easy production of xcex4 ferrite. The martensite steel of the present invention uses 0.05% to 2.0% by weight of molybdenum, preferentially 0.05% to 0.5%, more preferentially 0.1% to 0.3%.
Tungsten(W) is more effective than molybdenum to suppress cohesion of M23C6 carbide particles, to solidify and strengthen the matrix and to increase the high-temperature strength of the steel. The martensite steel of the present invention requires a minimum of 0.5% by weight of tungsten. 5% or above by weight of tungsten causes easy production of xcex4 ferrite and laves phases, which decreases the high-temperature strength of the steel. The martensite steel of the present invention preferentially uses 1.0% to 3.0% by weight of tungsten.
Vanadium(V) is effective to separate vanadium carbides and increase the high-temperature strength of the steel. The martensite steel of the present invention requires a minimum of 0.05% by weight of vanadium. More than 0.3% by weight of vanadium settles excessive carbons in the steel, separates less M23C6 carbides, and reduces the high-temperature strength of the steel. The martensite steel of the present invention preferentially uses 0.05% to 0.3% by weight of vanadium, more preferentially 0.10% to 0.25%.
At least one of niobium(Nb) and tantalum(Ta) works to produce NbC or TaC and make the crystal particles smaller. Part of niobium Nb and tantalum Ta dissolves during quenching and separates NbC and TaC in the tempering process. This increases the high-temperature strength of the steel. The martensite steel of the present invention requires a minimum of 0.01% by weight of niobium Nb and tantalum Ta. More than 0.20% by weight of niobium Nb and tantalum Ta as well as vanadium settles excessive carbons in the steel, separates less M23C6 carbides, and reduces the high-temperature strength of the steel. The martensite steel of the present invention preferentially uses 0.01% to 0.20% by weight of niobium Nb and tantalum Ta, more preferentially 0.04% to 0.13%.
Cobalt (Co) is an important element which distinguishes the present invention from the conventional inventions. This invention uses 0.5% by weight or above of cobalt and dramatically increases the high-temperature strength of the steel. This effect is assumed to be caused by an interaction of cobalt and tungsten and is a symptom characteristic to the alloy containing 0.5% by weight or more of tungsten in accordance with the present invention. An excessive addition of cobalt (10% by weight or above) will decrease the ductility of the steel and is not preferable. The martensite steel of the present invention preferentially uses more than 1.0% to 4.0% or under by weight of cobalt, more preferentially 1.5% to 3.5%.
Nitrogen(N) is effective to separate nitride of vanadium and to increase the high-temperature strength by the IS effect (interaction of invasion type solid solution elements and substitution type solid solution elements) together with molybdenum(Mo) and tungsten W in the solid solution status. The martensite steel of the present invention requires a minimum of 0.01% by weight of nitrogen N. More than 0.1% by weight of nitrogen reduces the ductility of the steel. The martensite steel of the present invention preferentially uses 0.01% to 0.1% by weight of nitrogen, more preferentially 0.01% to 0.04%.
Silicon(Si) accelerates production of a laves phase and reduces the ductility of the steel by segregation of grain boundaries. Therefore the quantity of silicon in the steel must be 0.15% by weight or under. more preferentially 0.01% or under. However, an extremely small quantity of silicon (0.03% by weight or above) added as a deoxidizer gives a preferable high-temperature characteristic together with an aluminum deoxidizer (to be explained later).
Aluminum(Al) plays the most important role in the preset invention. 0.0005% by weight of aluminum is added as a deoxidizer and an agent to make grains boundary smaller.
Further, aluminum working as a strong nitride producing element settles nitrogen which has an effect on creeping and consequentially decreases a long-term creep strength (10,000 hours or longer) at a high temperature of 625 degrees C to 700 degrees C (particularly when more than 0.006% by weight of aluminum is added). Furthermore, aluminum can accelerate separation of a laves phase which is a fragile intermetallic compound having tungsten as its main ingredient to the grain boundaries and reduce the creep rupture strength for a long term. Particularly, when crystal particles are made smallest, the laves phase is separated continuously to the grain boundaries. Therefore, the quantity of aluminum in the martensite steel of the present invention must be up to 0.006% by weight. The martensite steel of the present invention preferentially uses 0.001% to 0.004% by weight of aluminum. This is more effective when the quantity of tungsten in the steel is greater (1.5% to 3.0% by weight).
Boron(B) works to strengthen grain boundaries. Further, boron dissolves into M23C6 carbide and prevents M23C6 carbide particles from growing bigger. This increases the high-temperature strength of the steel. A minimum of 0.001% by weight boron in the steel is very effective but more than 0.030% by weight of boron reduces the weldability and forging ability of the steel. Therefore, the martensite steel of the present invention uses 0.001% to 0.030% by weight of boron, more preferentially 0.002% to 0.025%.
The preferential equivalent of chrome expressed by a formula below is 4% to 10.5% by weight, more particularly 6.5% to 9.5%.
Chrome equivalent=xe2x88x9240xc3x97C %xe2x88x9230xc3x97N %xe2x88x922xc3x97Mn %xe2x88x924xc3x97Ni %+Cr %+6xc3x97Si %+4xc3x97Mo %+1.5xc3x97W %+11xc3x97V %+5xc3x97Nb %xe2x88x922xc3x97Co %
The rotor shaft of the present invention is produced by the steps of melting the ingot in a vacuum status, decarbonizing thereof in a vacuum status, ESR-dissolving, forging, heating thereof at 900 degrees C to 1150 degrees C, quenching at a cooling rate of 50 degrees C/hour to 600 degrees C/hour (in the center hole), tempering thereof at 500 degrees C to 620 degrees C, and tempering thereof again at 600 degrees C to 750 degrees C.
The rotor shaft in accordance with the present invention when applied to a ultra-super critical pressure thermal power plant can increase the steam temperature to 650 degrees C or above. This has a great effect to increase the thermal efficiency of a thermal power plant.